Glucose dehydrogenase

ABSTRACT

A modified pyrroloquinoline quinone glucose dehydrogenase that exhibits a high selectivity for glucose is provided. A modified pyrroloquinoline quinone glucose dehydrogenase is disclosed in which the amino acid residue G at Position 99 of a pyrroloquinoline quinone glucose dehydrogenase (PQQGDH) represented by SEQ ID NO: 1, or the amino acid residue G at Position 100 of the pyrroloquinoline quinone glucose dehydrogenase (PQQGDH) represented by SEQ ID NO: 3, is substituted by the amino acid sequence TGZN (where Z is SX, S, or N and X is any amino acid residue). The modified PQQGDH of the present invention may additionally comprise one or more mutations selected from the group consisting of Q192G, Q192A, or Q192S; L193X; E277X; A318X; Y367A, Y367F, or Y367W; G451C; and N452X (where X is any amino acid residue).

TECHNICAL FIELD

The present application claims priority based on Japanese Patent Application No. 2007-163858 (filed 21 Jun. 2007), the contents of which are hereby incorporated by reference.

The present invention relates to a pyrroloquinoline quinone-dependent glucose dehydrogenase (PQQGDH) and to its preparation and use in the glucose assay.

BACKGROUND ART

The blood glucose level is an important marker for diabetes. The use of PQQGDH has already been commercialized as one method for measuring the glucose concentration.

PQQGDH is a glucose dehydrogenase that employs pyrroloquinoline quinone as a coenzyme, and it catalyzes the oxidation of glucose with the production of gluconolactone. PQQGDH is known to occur as a membrane-bound enzyme and as a water-soluble enzyme. Membrane-bound PQQGDHs are single peptide proteins with molecular weights of approximately 87 kDa and are widely encountered in various Gram-negative bacteria. Water-soluble PQQGDH, on the other hand, has been identified in several strains of Acinetobacter calcoaceticus, and its structural gene was cloned and its amino acid sequence was determined (GenBank accession number X15871; Mol. Gen. Genet. (1989), 217: 430-436). The results of X-ray crystal structural analysis of water-soluble PQQGDH from Acinetobacter calcoaceticus have been reported and the higher order structure of the enzyme, and most importantly the active center, has been elucidated. (A. Oubrie et al., J. Mol. Biol., 289, 319-333 (1999); A. Oubrie et al., The EMBO Journal, 18(19), 5187-5194 (1999); A. Oubrie et al., PNAS, 96(21), 11787-11791 (1999)). Water-soluble PQQGDH from Acinetobacter baumannii has also been identified (GenBank accession number E28183).

PQQGDHs have a high oxidation activity for glucose and do not require oxygen as an electron acceptor because they are coenzyme-linked enzymes. As a result they are expected to find application in glucose assays and particularly as the recognition element of glucose sensors. A problem with PQQGDHs, however, is their low selectivity for glucose. In particular, PQQGDH also has a high activity for maltose, and thus accurate assay is difficult in patients receiving a maltose-containing infusion solution. In this case, the apparent blood sugar level will be higher than the actual blood sugar level, which could lead to a risk of hypoglycemia caused by administering insulin to the patient based on the measured level. Accordingly, a PQQGDH that exhibits a higher selectivity for glucose versus maltose is desired for the enzyme used for measurement of the blood sugar level.

The present inventor has already reported several modified PQQGDHs that exhibit an increased selectivity for glucose (for example, WO 00/66744, Japanese Patent Application Laid-open No. 2001-346587, and WO 2004/005499), but a modified PQQGDH that exhibits an even higher selectivity and/or an even higher enzymatic activity is still required.

The reference documents cited herein are listed below. The contents of these documents are hereby incorporated by reference in its entirety. None of these documents are admitted to constitute a prior art of the present invention.

Patent Document 1: WO 00/66744 Patent Document 2: Japanese Patent Application Laid-open No. 2001-346587

-   Patent Document 3: WO 2004/005499 -   Nonpatent Document 1: Mol. Gen. Genet. (1989), 217:430-436 -   Nonpatent Document 2: A. Oubrie et al. (1999) J. Mol. Biol., 289,     319-333 -   Nonpatent Document 3: A. Oubrie et al. (1999) The EMBO Journal,     18(19), 5187-5194 -   Nonpatent Document 4: A. Oubrie et al. (1999) PNAS, 96(21),     11787-11791

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide a modified pyrroloquinoline quinone glucose dehydrogenase that exhibits a high selectivity for glucose.

The present inventor discovered that the selectivity for glucose is increased by the insertion, in a particular position in water-soluble PQQGDH, of a peptide fragment comprising 4 or 5 amino acids having a predetermined sequence.

The present invention provides a modified pyrroloquinoline quinone glucose dehydrogenase in which the amino acid residue G at Position 99 of a pyrroloquinoline quinone glucose dehydrogenase (PQQGDH) represented by SEQ ID NO: 1 or the amino acid residue G at Position 100 of the PQQGDH represented by SEQ ID NO: 3 is substituted by the amino acid sequence TGZN (where Z is SX, S, or N and X is any amino acid residue), and wherein from 1 to 10 of amino acid residues at Positions 1 to 98 and amino acid residues at Positions 100 to 478 of SEQ ID NO: 1, or amino acid residues at Positions 1 to 99 and amino acid residues at Positions 101 to 480 of SEQ ID NO: 3, may be substituted by any other amino acid residue(s). Z in the modified pyrroloquinoline quinone glucose dehydrogenase of the present invention is preferably SX and is particularly preferably SN.

Preferably, the modified pyrroloquinoline quinone glucose dehydrogenase of the present invention further comprises one or more mutations selected from the group consisting of the following amino acid substitutions:

Q192G, Q192A, or Q192S;

L193X (where X is any amino acid residue); E277X (where X is any amino acid residue); A318X (where X is any amino acid residue);

Y367A, Y367F, or Y367W; G451C; and

N452X (where X is any amino acid residue).

In another aspect the present invention provides a gene coding for the modified pyrroloquinoline quinone glucose dehydrogenase according to the present invention, a recombinant vector comprising the gene, and a transformant or transfectant that has been transformed with the recombinant vector. The present invention further provides a method of preparing the modified pyrroloquinoline quinone glucose dehydrogenase, comprising culturing a transformant that was transformed by a recombinant vector comprising a gene coding for the modified pyrroloquinoline quinone glucose dehydrogenase according to the present invention; and recovering the modified pyrroloquinoline quinone glucose dehydrogenase from the culture.

In an additional aspect the present invention provides a glucose assay kit comprising the modified pyrroloquinoline quinone glucose dehydrogenase according to the present invention. The present invention additionally provides an enzyme electrode comprising the modified pyrroloquinoline quinone glucose dehydrogenase according to the present invention, as well as a glucose sensor comprising this enzyme electrode as a working electrode.

The modified pyrroloquinoline quinone glucose dehydrogenase of the present invention exhibits a high selectivity for glucose as well as a high glucose oxidation activity, and can therefore be used for the highly selective and highly sensitive assay of glucose.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the enzymatic activity of the modified PQQGDH of the present invention for glucose and maltose.

PREFERRED EMBODIMENT OF THE INVENTION Structure of the Modified PQQGDH

The modified pyrroloquinoline quinone glucose dehydrogenase of the present invention is characterized in that the amino acid residue G at Position 99 of the water-soluble PQQGDH shown by SEQ ID NO: 1 or the amino acid residue G at Position 100 of the PQQGDH shown by SEQ ID NO: 3 is substituted by the amino acid sequence TGZN (where Z is SX, S, or N and X is any amino acid residue). As used herein, the position of an amino acid in the amino acid sequence of the water-soluble PQQGDHs is numbered by assigning 1 to the initiation methionine.

The PQQGDH represented by SEQ ID NO: 1 is a PQQGDH from Acinetobacter calcoaceticus (GenBank accession number X15871) and the PQQGDH represented by SEQ ID NO: 3 is a PQQGDH from Acinetobacter baumannii (GenBank accession number E28183). These PQQGDHs have an approximately 92% homology at the level of the amino acid sequence. The alignment of the two sequences is shown below.

TABLE 1 _aln.pos          10        20        30        40        50        60 Calcoace  MNKHLLAKIALLSAVQLVTL-SAFADVPLTPSQFAKAKSENFDKKVILSNLNKPHALLWGPDNQIWLT Baumannii  MNKHLLAKITLLGAAQLFTFHTAFADIPLTPAQFAKAKTENFDKKVILSNLNKPHALLWGPDNQIWLT _consrvd  ************ * ** *   * **** ****** ***************************** _aln.p  70        80        90       100       110       120       130 calocoace  ERATGKILRVNPESGSVKTVFQVPEIVNDADGQNGLLGFAFHPDFKNNPYIYISGTFKNPKSTD Baumannii  ERATGKILRVNPVSGSAKTVFQVPEIVSDADGQNGLLGFAFHPDFKHNPYIYISGTFKNPKSTD _consrvd  ************ *** ********** ****************** ***************** _aln.pos   140       150       160       170       180       190       200 calcoace  KELPNQTIIRRYTYNKSTDTLEKPVDLLAGLPSSKDHQSGRLVIGPDQKIYYTIGDQGRNQLAYLFLP Baumannii  KELPNQTIIRRYTYNKTTDTFEKPIDLIAGLPSSKDHQSGRLVIGPDQKIYYTIGDQGRNQLAYLFLS _consrvd  **************** *** *** ** *************************************** _aln.pos     210       220       230       240       250       260       270 calcoace  NQAQHTPTQQELNGKDYHTYWGKVLRLNLDGSIPKDNPSFNGVVSHIYTLGHRNPQGLAFTPNGKLLQ Baumannii  NQAQHTPTQQELNSKDYHTYMGKVLRLNLDGSIPKDNPSFNGVVSHIYTLGHRNPQGLAFAPNGKLLQ _consrvd  ************* ********************************************** ******* _aln.pos       280       290       300       310       320       330       340 calcoace  SEQGPNSDDEINLIVKGGNYGWPNVAGYKDDSGYAYANYSAAANK-SIKDLAQNGVKVAAGVPVTKES Baumannii  SEQGPNSDDEINLVLKGGNYGWPNVAGYKDDSGYAYANYSAATNKSQIKDLAQNGIKVATGVPVTKES _consrvd  *************  *************************** **  ******** *** ******** _aln.pos         350       360       370       380       390       400 calcoace  EWTGKNFVPPLKTLYTVQDTYNYNDPTCGEMTYICWPTVAPSSAYVYKGGKKAITGWENTLLVPSLKR Baumannii  EWTGKNFVPPLKTLYTVQDTYNYNDPTCGEMAYICWPTVAPSSAYVYTGGKKAIPGWENTLLVPSLKR _consrvd  ******************************* *************** ****** ************* _aln.p 410       420       430       440       450       460       470 calcoace  GVIFRIKLDPTYSTTYDDAVPMFKSNNRYRDVIASPDGNVLYVLTDTAGNVQKDDGSVTNTLENPGSL Baumannii  GVIFRIKLDPTYSTTLDDAIPMFKSNNRYRDVIASPEGNTLYVLTDTAGNVQKDDGSVTHTLENPGSL _consrvd  *************** *** **************** ** ******************* ******** _aln.pos   480 calcoace  IKFTYKAK Baumannii  IKFTYNGK _consrvd  *****  *

These two PQQGDHs have similar secondary structures, and it is known that the properties of enzymes, for example, the thermal stability and substrate specificity, are similarly changed by the substitution of corresponding amino acid residues.

Z in the modified pyrroloquinoline quinone glucose dehydrogenase of the present invention is preferably SX and particularly preferably is SN. Thus, the amino acid sequence TGSXN, preferably TGSNN, and particularly preferably TGSKN, TGSRN, or TGSWN is inserted in place of the amino acid residue G at Position 99 in SEQ ID NO: 1 or in place of the amino acid residue G at Position 100 in SEQ ID NO: 3. TGSN or TGNN is also preferably inserted in place of this amino acid residue G.

The modified PQQGDH of the present invention has a higher selectivity for glucose than naturally occurring water-soluble PQQGDH. The modified PQQGDH of the present invention preferably has a lower reactivity for maltose versus reactivity for glucose than does the wild type. Given the reactivity for glucose is 100%, preferably the activity for maltose is not more than 50%, more preferably not more than 30%, even more preferably not more than 20%, and most preferably not more than 10%.

The modified PQQGDH of the present invention may have other mutations in addition to the mutation in the amino acid residue at Position 99 of SEQ ID NO: 1 or at Position 100 of SEQ ID NO: 3. For example, one or more, for example, from 1 to 10, of the amino acid residues at Positions 1 to 98 or at Positions 100 to 478 of SEQ ID NO: 1, or the amino acid residues at Positions 1 to 99 or at Positions 101 to 480 of SEQ ID NO: 3, may be substituted by any other amino acid residue.

In a preferred embodiment of the present invention, the modified PQQGDH of the present invention has one or more mutations, preferably from 1 to 10 mutations, for example, 1, 2, 3, 4, 5, or 6 mutations, selected from the group consisting of

Q192G, Q192A, or Q192S;

L193X (where X is any amino acid residue); E277X (where X is any amino acid residue); A318X (where X is any amino acid residue);

Y367A, Y367F, or Y367W; G451C; and

N452X (where X is any amino acid residue). Wherein, the “Q192G” designation, for example, indicates that the glutamine at Position 192 in SEQ ID NO: 1, or the corresponding glutamine at Position 193 in SEQ ID NO: 3, is substituted by glycine. The other substitutional mutations are also indicated in the same manner.

A particularly preferred embodiment of the present invention is a modified pyrroloquinoline quinone glucose dehydrogenase comprising any of the following combinations of amino acid mutations:

G99 (TGSXN)+Q192G+L193E; G99 (TGSXN)+Q192S+N452P; G99 (TGSXN)+Q192G+L193E+N452P; G99 (TGSXN)+Q192S+N452P; G99 (TGSNN)+N452P; G99 (TGSNN)+Q192G+L193E+N452P; G99 (TGSNN)+Q192S+N452P; G99 (TGSNN)+Q192G+N452P; G99 (TGSNN)+L193E+N452P; G99 (TGSNN)+Q192S+L193M+N452P; G99 (TGSNN)+A318Y+N452P; G99 (TGSNN)+Q192G; G99 (TGSNN)+Q192S; G99 (TGSNN)+Q192A; G99 (TGSNN)+Q192G+L193E; G99 (TGSNN)+Q192S+L193X; G99 (TGSNN)+Q192S+L193M; G99 (TGSNN)+Q192S+L193T; G99 (TGSNN)+Q192S+E277X; G99 (TGSNN)+Q192S+N452X; G99 (TGSNN)+Q192S+L193X+A318Y+N452P; G99 (TGSNN)+Q192S+A318X+N452P; G99 (TGSNN)+Q192G+L193E+A318X+N452P; G99 (TGSKN)+Q192S+N452P; G99 (TGSRN)+Q192S+N452P; G99 (TGSWN)+Q192S+N452P; G99 (TGSN)+Q192S+N452P; G99 (TGSN)+Q192G+L193E; G99 (TGSN)+Q192S+L193M; G99 (TGNN)+Q192S+N452P; G99 (TGNN)+Q192G+L193E; or G99 (TGNN)+Q192S+L193M.

Wherein X is any amino acid reside, for example, G99 (TGSXN) indicates that the G at Position 99 in the PQQGDH represented by SEQ ID NO: 1 is substituted by TGSXN.

Another preferred embodiment of the present invention is a modified pyrroloquinoline quinone glucose dehydrogenase comprising any of the following amino acid mutations:

G99 (TGSXN)+Q192G+L193E; G99 (TGSXN)+Q192S+N452P; G99 (TGSNN)+Q192G+L193E+N452P; G99 (TGSNN)+Q192S+L193M+N452P; G99 (TGSNN)+Q192G+L193E+A318K+N452P; or G99 (TGSNN)+Q192G+L193E+A318Q+N452P.

WO 04/005499 discloses that the glutamine residue at Position 192, the leucine residue at Position 193, and the asparagine residue at Position 452 are involved in substrate recognition and binding by PQQGDH. In general, however, it is completely unpredictable how the substrate specificity and enzymatic activity will change when mutations are simultaneously introduced in amino acid residues present in different domains. A complete loss of enzymatic activity may even occur in some cases. Accordingly, it was totally unpredictable if the selectivity for glucose would be further enhanced by the simultaneous introduction of the above-indicated substitutions and an insertion mutation at the amino acid residue at Position 99 in SEQ ID NO: 1 or the amino acid residue at Position 100 in SEQ ID NO: 3.

Method of Producing Modified PQQGDH

The sequence of the gene coding for naturally occurring water-soluble PQQGDH from Acinetobacter calcoaceticus is shown in SEQ ID NO: 2, while the sequence of the gene coding for naturally occurring water-soluble PQQGDH from Acinetobacter baumannii is shown in SEQ ID NO:4. A gene coding for the modified PQQGDH of the present invention can be constructed by replacing the nucleotide sequence coding for the amino acid residue targeted for substitution in the gene coding for naturally occurring water-soluble PQQGDH with a nucleotide sequence coding for the desired amino acid residue. Methods for such a site-specific sequence substitution are well known in the art; for example, by PCR using suitably designed primers, as is described in the examples provided below.

The thus obtained mutant gene is inserted in an expression vector (for example, a plasmid), which is then transformed into a suitable host (for example, E. coli). A large number of vector/host systems for the expression of foreign protein are known in the art. A variety of hosts, for example, bacteria, yeast, cultured cells, and so forth are available.

In addition, a portion of other amino acid residues in the modified PQQGDH of the present invention may also be deleted or substituted, and other amino acid residues may be added, insofar as it has a desired glucose dehydrogenase activity. Various methods for site-specific nucleotide sequence substitution are well known in the art.

The modified PQQGDH-expressing transformant obtained as described above is then cultured, and the cells are recovered from the culture fluid by, for example, centrifugation. The recombinant protein present in the periplasmic compartment is subsequently released into culture medium by grinding the cells, for example, with a French press, or by osmotic shock. Ultracentrifugation is thereafter carried out to obtain a water-soluble fraction containing the modified PQQGDH. Alternatively, through the use of a suitable host-vector system, the expressed modified PQQGDH can be secreted into the culture fluid. The modified PQQGDH of the present invention can be isolated by purifying the water-soluble fraction by, for example, ion-exchange chromatography, affinity chromatography, HPLC, and so forth.

Method of Measuring the Enzymatic Activity

The modified PQQGDH of the present invention has the ability to catalyze the oxidation of glucose with PQQ as its coenzyme, with the production of gluconolactone. To measure the enzymatic activity, the quantity of PQQ that is reduced accompanying the PQQGDH-mediated glucose oxidation can be quantitated by the color reaction of a redox dye. For example, phenazine methosulfate (PMS), 2,6-dichlorophenolindophenol (DCIP), potassium ferricyanide, ferrocene, and so forth, can be used as the chromogenic reagent.

Selectivity for Glucose

The selectivity for glucose exhibited by the modified PQQGDH of the present invention can be evaluated by measuring the enzymatic activity in the manner described above using various sugars as substrates, e.g., 2-deoxy-D-glucose, mannose, allose, 3-o-methyl-D-glucose, galactose, xylose, lactose, maltose, and so forth, and determining the relative activity with reference to the activity using glucose as the substrate.

The modified PQQGDH of the present invention provides a higher selectivity for glucose than the wild-type enzyme and in particular has a higher reactivity for glucose than for maltose. Accordingly, an assay kit or enzyme sensor constructed using the modified PQQGDH of the present invention will exhibit a high selectivity with regard to glucose measurement and offers the advantage of high-sensitivity glucose detection even when a maltose-containing sample is used.

Glucose Assay Kit

Another aspect of the present invention is a glucose assay kit comprising a modified PQQGDH according to the present invention. The glucose assay kit of the present invention comprises the modified PQQGDH according to the present invention in a quantity sufficient for conducting at least one assay. In addition to the modified PQQGDH of the present invention, the kit will typically comprise a buffer required for the assay, a mediator, a glucose reference solution for constructing a calibration curve, and instructions for use. The modified PQQGDH according to the present invention can be provided in various forms, for example, as a freeze-dried reagent or as a solution in a suitable storage solution. The modified PQQGDH of the present invention is preferably provided in a form of holoenzyme, but can also be provided as the apoenzyme and then converted into the holoenzyme before use.

Glucose Sensor

Additional aspect of the present invention is an enzyme electrode that carries a modified PQQGDH according to the present invention, and a glucose sensor comprising the enzyme electrode. For example, a carbon electrode, gold electrode, or platinum electrode can be used as the electrode, and the enzyme according to the present invention is immobilized on the electrode. The immobilization method can be exemplified by the use of a crosslinking reagent; enclosure in a polymer matrix; coating with a dialysis film; use of a photocrosslinking polymer, electroconductive polymer, or redox polymer; immobilization in a polymer or adsorptive immobilization on the electrode together with an electron mediator such as ferrocene and its derivatives; and the use of combinations of the preceding. The modified PQQGDH of the present invention is preferably immobilized on the electrode in a holoenzyme form, but can also be immobilized in an apoenzyme form with the PQQ being provided in a separate layer or in a solution. Typically, the modified PQQGDH of the present invention is immobilized on a carbon electrode via glutaraldehyde followed by treatment with an amine group-containing reagent to block the aldehyde groups of glutaraldehyde.

Measurement of the glucose concentration can be carried out as follows. Buffer solution is introduced into a thermostatted cell; PQQ, CaCl₂, and a mediator are added; and the cell is held at a constant temperature. Potassium ferricyanide, phenazine methosulfate, and so forth, may be used as the mediator. An electrode bearing the immobilized modified PQQGDH of the present invention is used as the working electrode, in combination with a counterelectrode (for example, a platinum electrode) and a reference electrode (for example, the Ag/AgCl electrode). A constant voltage is applied to the carbon electrode, and after the current becomes constant, a glucose-containing sample is added and the increase in the current is measured. The glucose concentration in the sample can be determined using a calibration curve constructed using glucose solutions of standard concentrations.

The contents of all the patents and reference documents explicitly cited herein are hereby incorporated by reference in its entirety.

The present invention is described in detail based on the following examples, but is not limited to these examples.

Example 1 Construction of Modified PQQGDH-Encoding Genes

Mutation was introduced into the structural gene for water-soluble PQQGDH from Acinetobacter calcoaceticus, which is represented by SEQ ID NO: 2. In brief, PCR was carried out using a full length forward primer for the wild-type water-soluble PQQGDH and a mutagenic reverse primer, and another PCR was carried out using a full length reverse primer and a mutagenic forward primer. These PCR products were mixed and PCR was run using a full length forward primer and reverse primer to obtain a gene coding for the mutated full length PQQGDH. The product was sequenced to conform that the desired mutation was correctly introduced.

The sequence of the primers for full length amplification were as follows.

forward: (SEQ ID NO: 5) AACAGACCATGGATAAACATTTATTGGC reverse: (SEQ ID NO: 6) ACAGCCAAGCTTTTACTTAGCCTTATAGG

The sequences of the mutagenic primers (F: forward, R: reverse) are shown in the following table.

TABLE 2 SEQ ID Mutation Primer Sequence NO TGNN ACTGGAAATAATCAGAATGGTTTATTAGGTTTT F 7 CTGATTATTTCCAGTATCAGCATCATTGACAAT R 8 TGQN ACTGGACAGAATCAGAATGGTTTATTAGGTTTT F 9 CTGATTCTGTCCAGTATCAGCATCATTGACAAT R 10 TGSN ACTGGAAGCAATCAGAATGGTTTATTAGGTTTT F 11 CTGATTGCTTCCAGTATCAGCATCATTGACAAT R 12 TGGN ACTGGAGGTAATCAGAATGGTTTATTAGGTTTT F 13 CTGATTACCTCCAGTATCAGCATCATTGACAAT R 14 TGSSN GATACTGGAAGCAGCAATCAGAATGGTTTATTA F 15 ATTGCTGCTTCCAGTATCAGCATCATTGACAAT R 16 TGWN ACTGGATGGAATCAGAATGGTTTATTAGGTTTT F 17 CTGATTCCATCCAGTATCAGCATCATTGACAAT R 18 TGFN ACTGGATTTAATCAGAATGGTTTATTAGGTTTT F 19 CTGATTAAATCCAGTATCAGCATCATTGACAAT R 20 TGDN ACTGGAGATAATCAGAATGGTTTATTAGGTTTT F 21 CTGATTATCTCCAGTATCAGCATCATTGACAAT R 22 TGSHN GATACTGGAAGCCATAATCAGAATGGTTTATTA F 23 ATTATGGCTTCCAGTATCAGCATCATTGACAAT R 24 TGSLN GATACTGGAAGCTTAAATCAGAATGGTTTATTA F 25 ATTTAAGCTTCCAGTATCAGCATCATTGACAAT R 26 TGSVN GATACTGGAAGCGTCAATCAGAATGGTTTATTA F 27 ATTGACGCTTCCAGTATCAGCATCATTGACAAT R 28 TGSQN GATACTGGAAGCCAAAATCAGAATGGTTTATTA F 29 ATTTTGGCTTCCAGTATCAGCATCATTGACAAT R 30 TGSEN GATACTGGAAGCGAAAATCAGAATGGTTTATTA F 31 ATTTTCGCTTCCAGTATCAGCATCATTGACAAT R 32 TGSDN GATACTGGAAGCGATAATCAGAATGGTTTATTA F 33 ATTATCGCTTCCAGTATCAGCATCATTGACAAT R 34 TGSPN GATACTGGAAGCCCTAATCAGAATGGTTTATTA F 35 ATTAGGGCTTCCAGTATCAGCATCATTGACAAT R 36 TGSTN GATACTGGAAGCACAAATCAGAATGGTTTATTA F 37 ATTTGTGCTTCCAGTATCAGCATCATTGACAAT R 38 TGSIN GATACTGGAAGCATTAATCAGAATGGTTTATTA F 39 ATTAATGCTTCCAGTATCAGCATCATTGACAAT R 40 TGSAN GATACTGGAAGCGCTAATCAGAATGGTTTATTA F 41 ATTAGCGCTTCCAGTATCAGCATCATTGACAAT R 42 TGSWN GATACTGGAAGCTGGAATCAGAATGGTTTATTA F 43 ATTCCAGCTTCCAGTATCAGCATCATTGACAAT R 44 TGSGN GATACTGGAAGCGGTAATCAGAATGGTTTATTA F 45 ATTACCGCTTCCAGTATCAGCATCATTGACAAT R 46 TGSFN GATACTGGAAGCTTTAATCAGAATGGTTTATTA F 47 ATTAAAGCTTCCAGTATCAGCATCATTGACAAT R 48 TGSYN GATACTGGAAGCTATAATCAGAATGGTTTATTA F 49 ATTATAGCTTCCAGTATCAGCATCATTGACAAT R 50 TGSCN GATACTGGAAGCTGCAATCAGAATGGTTTATTA F 51 ATTGCAGCTTCCAGTATCAGCATCATTGACAAT R 52 TGSMN GATACTGGAAGCATGAATCAGAATGGTTTATTA F 53 ATTCATGCTTCCAGTATCAGCATCATTGACAAT R 54 TGSKN GATACTGGAAGCAAAAATCAGAATGGTTTATTA F 55 ATTTTTGCTTCCAGTATCAGCATCATTGACAAT R 56 TGSRN GATACTGGAAGCCGTAATCAGAATGGTTTATTA F 57 ATTACGGCTTCCAGTATCAGCATCATTGACAAT R 58 TGSNN GATACTGGAAGCAATAATCAGAATGGTTTATTA F 59 ATTATTGCTTCCAGTATCAGCATCATTGACAAT R 60

Other substitutional mutations, e.g., mutations such as Q192G and L193E were introduced using conventional site-specific mutagenesis methods as described in WO 00/66744. In addition, modified PQQGDHs that incorporated a combination of mutations at a plurality of sites were prepared by site-specific mutagenesis using a plurality of primers that corresponded to the individual mutations or were prepared by a recombinant method using restriction enzymes.

Example 2 Production of Modified PQQGDH

The gene coding for wild-type PQQGDH or the gene coding for the modified PQQGDH constructed as described above was inserted in the multicloning site of an E. coli expression vector pTrc99A (Pharmacia), and the constructed plasmid was transformed into E. coli. The transformant was cultured with shaking overnight at 37° C. in 450 mL L-broth containing 50 μg/mL ampicillin and 30 μg/mL chloramphenicol. The culture was inoculated into 7 L of L-broth containing 1 mM CaCl₂ and 500 μM PQQ. At approximately 3 hours after the start of cultivation, isopropylthiogalactoside was added at a final concentration of 0.3 mM and continued cultivation for 1.5 hours. The cells were recovered from the culture medium by centrifugation (5000×g, 10 minutes, 4° C.) and suspended in 150 μL 10 mM MOPS (pH 7.0). Glass beads (0.105 to 0.125 mm for bacteria) were added in an amount that was ½ to ⅔ of the volume and vortexed for 20 minutes at 4° C. 1 mL 10 mM MOPS (pH 7.0) was added and centrifuged (15,000 rpm, 20 minutes, 4° C.). The supernatant was collected and used as a crude enzyme preparation in the following examples.

Example 3 Measurement of the Enzymatic Activity

900 μl of a mixture of 10 mM MOPS (pH 7.0)+1 mM PQQ+1 mM CaCl₂ holoenzyme conversion solution was added to 100 μl of the crude enzyme preparation of the wild-type PQQGDH or the modified PQQGDH obtained in Example 2 and left stand (room temperature, in the dark, at least 30 minutes) to allow for conversion to the holoenzyme. After completion of conversion to the holoenzyme, the solution was diluted 10× to prepare an enzyme test solution. To 150 μl of the enzyme test solution was added 50 μl of a given concentration of the substrate (glucose or maltose), 0.6 mM phenazine methosulfate (PMS), and 0.3 mM 2,6-dichlorophenolindophenol (DCIP, final concentrations in each case), and incubated at room temperature in the total volume of 200 μL. Glucose and maltose were used as the substrates at final concentrations of 0, 1, 2, 4, 10, 20, and 40 mM. The change in the DCIP absorbance at 600 nm was monitored with a spectrophotometer. The rate of decline in the absorbance was measured to determine the reaction rate of the enzyme. In the experiments described herein, 1 unit was designated as the enzymatic activity that reduces 1 μmol of DCIP in 1 minute with the molar absorption coefficient of 16.3 mM⁻¹ of DCIP at pH 7.0. The protein concentration was measured using a commercially available protein assay kit (BioRad).

Typical results are given in the following tables indicated by the ratio (%) of the activity for 4 mM maltose with reference to the activity for 4 mM glucose.

TABLE 3 TGSXN Wild Type 79 Q192G + L193E 9.0 TGSAN 54 TGSCN 47 TGSDN 60 TGSEN 51 TGSGN 52 TGSHN 46 TGSIN 42 TGSKN 43 TGSMN 43 TGSNN 42 TGSPN 47 TGSQN 42 TGSRN 43 TGSSN 57 TGSTN 47 TGSVN 39 TGSWN 33 TGSFN 58 TGSLN 49 TGSYN 63

TABLE 4 TGSXN + Q192G + L193E Wild Type 79 Q192G + L193E 9.0 TGSGN + Q192G + L193E 6.7 TGSMN + Q192G + L193E 5.7 TGSNN + Q192G + L193E 4.2 TGSPN + Q192G + L193E 6.6 TGSQN + Q192G + L193E 5.2 TGSRN + Q192G + L193E 6.0 TGSSN + Q192G + L193E 9.3 TGSTN + Q192G + L193E 7.4 TGSVN + Q192G + L193E 6.9 TGSAN + Q192G + L193E 15 TGSDN + Q192G + L193E 16 TGSEN + Q192G + L193E 20 TGSHN + Q192G + L193E 9.8 TGSIN + Q192G + L193E 20 TGSKN + Q192G + L193E 5.7 TGSCN + Q192G + L193E 17 TGSWN + Q192G + L193E 7.6

TABLE 5 TGSNN + Q192S + E277X TGSNN + Q192S 12 TGSNN + Q192S + E277A 20 TGSNN + Q192S + E277D 26 TGSNN + Q192S + E277I 15 TGSNN + Q192S + E277T 22 TGSNN + Q192S + E277Q 16 TGSNN + Q192S + E277V 23 TGSNN + Q192S + E277W 38 TGSNN + Q192S + E277Y 27 TGSNN + Q192S + E277C 12 TGSNN + Q192S + E277F 66 TGSNN + Q192S + E277G 34 TGSNN + Q192S + E277H 39 TGSNN + Q192S + E277K 29 TGSNN + Q192S + E277L 76 TGSNN + Q192S + E277M 64 TGSNN + Q192S + E277N 56 TGSNN + Q192S + E277S 13 TGSNN + Q192S + E277R 24 TGSNN + Q192S + E277P 39

TABLE 6 TGSNN + Q192S + N452X TGSNN + Q192S + N452A 8.9 TGSNN + Q192S + N452C 12 TGSNN + Q192S + N452D 20 TGSNN + Q192S + N452E nd TGSNN + Q192S + N452F 10 TGSNN + Q192S + N452G 14 TGSNN + Q192S + N452H 13 TGSNN + Q192S + N452I 14 TGSNN + Q192S + N452K 18 TGSNN + Q192S + N452L 7.8 TGSNN + Q192S + N452M 11 TGSNN + Q192S + N452P 1.9 TGSNN + Q192S + N452Q nd TGSNN + Q192S + N452R 27 TGSNN + Q192S + N452S 11 TGSNN + Q192S + N452T 14 TGSNN + Q192S + N452V 8.8 TGSNN + Q192S + N452W 79 TGSNN + Q192S + N452Y 9.3

TABLE 7 TGSNN + Q192G/S/A Wild Type 79 Q192G + L193E 9.0 TGSNN + Q192G + L193E 4.2 Q192G 20 TGSNN + Q192G 9.1 Q192A 15 TGSNN + Q192A 7.8 Q192S 28 TGSNN + Q192S 12

TABLE 8 TGSNN + Q192S + L193X TGSNN + Q192S 12 TGSNN + Q192G + L193E 4.2 TGSNN + Q192S + L193G 6.8 TGSNN + Q192S + L193A 7.3 TGSNN + Q192S + L193K 3.7 TGSNN + Q192S + L193R 7.7 TGSNN + Q192S + L193H 8.3 TGSNN + Q192S + L193D 6.8 TGSNN + Q192S + L193E 4.3 TGSNN + Q192S + L193N 8.9 TGSNN + Q192S + L193Q 11 TGSNN + Q192S + L193S 5.7 TGSNN + Q192S + L193T 6.3 TGSNN + Q192S + L193Y 7.4 TGSNN + Q192S + L193C 9.3 TGSNN + Q192S + L193M 4.0 TGSNN + Q192S + L193F 7.8 TGSNN + Q192S + L193W 8.2 TGSNN + Q192S + L193V 7.0 TGSNN + Q192S + L193I 6.9 TGSNN + Q192S + L193P 6.1

TABLE 9 Wild Type 79 TGSNN 42 TGSNN + Q192S 12 TGSNN + N452P 14 TGSNN + Q192G + L193E 4.2 TGSNN + Q192S + L193M 4.0 TGSNN + Q192S + L193T 5.8 TGSNN + Q192S + N452P 1.9 TGSNN + Q192G + N452P 1.6 TGSNN + Y367A + N452P 50 TGSNN + Y367F + N452P 12 TGSNN + Y367W + N452P 15 TGSNN + Q192S + Y367A + N452P nd TGSNN + Q192S + Y367F + N452P 2.7 TGSNN + Q192S + Y367W + N452P 2.0 TGSNN + L193E + N452P 1.0 TGSNN + Q192G + L193E + N452P 0.6

TABLE 10 Wild Type 79 G451C 74 TCSRN 33 TCSRN + G451C 44 N452P 30 TGSNN + Q192S + L193M 4.0 TGSNN + Q192S + L193M + N452P 0.8 TGSNN + Q192G + L193E + E277K 2.7 TGSNN + Q192G + L193E + N452P 0.6 TGSNN + N452P 14 TGSNN + A318Y + N452P 7.8

TABLE 11 TGSXN + Q192S + N452P TGSAN + Q192S + N452P 7.6 TGSCN + Q192S + N452P 3.9 TGSDN + Q192S + N452P 3.6 TGSFN + Q192S + N452P 26 TGSGN + Q192S + N452P 3.6 TGSLN + Q192S + N452P 2.2 TGSMN + Q192S + N452P 2.5 TGSNN + Q192S + N452P 1.9 TGSPN + Q192S + N452P 19 TGSSN + Q192S + N452P 7.4 TGSTN + Q192S + N452P 3.8 TGSVN + Q192S + N452P 3.7 TGSIN + Q192S + N452P 3.4 TGSWN + Q192S + N452P 1.2 TGSYN + Q192S + N452P 1.4 TGSEN + Q192S + N452P 5.8 TGSHN + Q192S + N452P 4.0 TGSQN + Q192S + N452P 5.1 TGSRN + Q192S + N452P 1.5

TABLE 12 TGSXN + Q192G + L193E + N452P TGSAN + Q192G + L193E + N452P 4.1 TGSFN + Q192G + L193E + N452P nd TGSHN + Q192G + L193E + N452P 2.3 TGSIN + Q192G + L193E + N452P 1.6 TGSLN + Q192G + L193E + N452P 1.4 TGSNN + Q192G + L193E + N452P 0.6 TGSQN + Q192G + L193E + N452P 8.4 TGSTN + Q192G + L193E + N452P nd TGSVN + Q192G + L193E + N452P 2.3 TGSCN + Q192G + L193E + N452P 44 TGSDN + Q192G + L193E + N452P 7.2 TGSMN + Q192G + L193E + N452P 7.3 TGSKN + Q192G + L193E + N452P 2.4 TGSPN + Q192G + L193E + N452P 5.4 TGSRN + Q192G + L193E + N452P 3.1 TGSSN + Q192G + L193E + N452P 5.2 TGSWN + Q192G + L193E + N452P 2.7 TGSYN + Q192G + L193E + N452P 3.1

TABLE 13 TGSXN + Q192S + N452P TGSNN 42 TGSKN 43 TGSRN 43 TGSWN 33 TGSNN + Q192S 12 TGSKN + Q192S 14 TGSRN + Q192S 8.1 TGSWN + Q192S 7.2 TGSNN + N452P 14 TGSKN + N452P 16 TGSRN + N452P 15 TGSWN + N452P 10 TGSNN + Q192S + N452P 1.9 TGSKN + Q192S + N452P 1.8 TGSRN + Q192S + N452P 1.5 TGSWN + Q192S + N452P 1.2

TABLE 14 TGSNN + Q192S + L193X + A318Y + N452P TGSNN + Q192S + L193M + N452P 0.8 TGSNN + Q192S + A318Y + N452P 1.5 TGSNN + Q192S + L193G + A318Y + N452P 1.0 TGSNN + Q192S + L193A + A318Y + N452P 8.8 TGSNN + Q192S + L193K + A318Y + N452P 0.5 TGSNN + Q192S + L193R + A318Y + N452P 8.8 TGSNN + Q192S + L193H + A318Y + N452P 9.0 TGSNN + Q192S + L193D + A318Y + N452P 7.8 TGSNN + Q192S + L193E + A318Y + N452P 6.6 TGSNN + Q192S + L193N + A318Y + N452P 1.4 TGSNN + Q192S + L193Q + A318Y + N452P 2.2 TGSNN + Q192S + L193S + A318Y + N452P 1.0 TGSNN + Q192S + L193T + A318Y + N452P 8.5 TGSNN + Q192S + L193Y + A318Y + N452P 8.1 TGSNN + Q192S + L193C + A318Y + N452P 8.9 TGSNN + Q192S + L193M + A318Y + N452P 2.4 TGSNN + Q192S + L193F + A318Y + N452P 8.4 TGSNN + Q192S + L193W + A318Y + N452P 9.0 TGSNN + Q192S + L193V + A318Y + N452P 0.9 TGSNN + Q192S + L193I + A318Y + N452P 7.7 TGSNN + Q192S + L193P + A318Y + N452P 8.2

TABLE 15 TGSNN + Q192S + A318X + N452P TGSNN + Q192S + N452P 1.9 TGSNN + Q192S + A318G + N452P 2.6 TGSNN + Q192S + A318K + N452P 0.0 TGSNN + Q192S + A318R + N452P 3.0 TGSNN + Q192S + A318H + N452P 2.0 TGSNN + Q192S + A318D + N452P 2.1 TGSNN + Q192S + A318E + N452P 2.3 TGSNN + Q192S + A318N + N452P 2.4 TGSNN + Q192S + A318Q + N452P 2.8 TGSNN + Q192S + A318S + N452P nd TGSNN + Q192S + A318T + N452P 5.1 TGSNN + Q192S + A318Y + N452P 2.3 TGSNN + Q192S + A318C + N452P nd TGSNN + Q192S + A318M + N452P 3.0 TGSNN + Q192S + A318F + N452P 3.0 TGSNN + Q192S + A318W + N452P 0.1 TGSNN + Q192S + A318V + N452P 0.4 TGSNN + Q192S + A318L + N452P 3.0 TGSNN + Q192S + A318I + N452P 4.5 TGSNN + Q192S + A318P + N452P nd

TABLE 16 TGSNN + Q192G + L193E + A318X + N452P TGSNN + Q192G + L193E + N452P 0.6 TGSNN + Q192G + L193E + A318G + N452P 0.7 TGSNN + Q192G + L193E + A318K + N452P 0.7 TGSNN + Q192G + L193E + A318R + N452P 0.7 TGSNN + Q192G + L193E + A318H + N452P 0.8 TGSNN + Q192G + L193E + A318D + N452P 1.0 TGSNN + Q192G + L193E + A318E + N452P 0.9 TGSNN + Q192G + L193E + A318N + N452P 0.6 TGSNN + Q192G + L193E + A318Q + N452P 0.5 TGSNN + Q192G + L193E + A318S + N452P 0.6 TGSNN + Q192G + L193E + A318T + N452P nd TGSNN + Q192G + L193E + A318Y + N452P 1.0 TGSNN + Q192G + L193E + A318C + N452P 0.7 TGSNN + Q192G + L193E + A318M + N452P 0.5 TGSNN + Q192G + L193E + A318F + N452P 0.8 TGSNN + Q192G + L193E + A318W + N452P 1.0 TGSNN + Q192G + L193E + A318V + N452P nd TGSNN + Q192G + L193E + A318L + N452P 0.6 TGSNN + Q192G + L193E + A318I + N452P 1.9 TGSNN + Q192G + L193E + A318P + N452P nd

TABLE 17 TGSI(K, R, W)N TGSNN 42 TGSKN 43 TGSRN 43 TGSWN 33 TGSNN + Q192S 12 TGSKN + Q192S 14 TGSRN + Q192S 8.1 TGSWN + Q192S 7.2 TGSNN + N452P 14 TGSKN + N452P 16 TGSRN + N452P 15 TGSWN + N452P 10 TGSNN + Q192S + N452P 1.9 TGSKN + Q192S + N452P 1.8 TGSRN + Q192S + N452P 1.5 TGSWN + Q192S + N452P 1.2

TABLE 18 TGSN TGSN nd TGSN + N452P 38 TGSN + Q192S + N452P 9.5 TGSN + Q192G + L193E 11 TGSN + Q192G + L193E + N452P 2.5 TGSN + Q192S + L193M 16 TGSN + Q192S + L193M + N452P 5.5

TABLE 19 TGNN TGNN 38 TGNN + N452P 22 TGNN + Q192S + N452P 5.3 TGNN + Q192G + L193E 11 TGNN + Q192G + L193E + N452P 2.1 TGNN + Q192S + L193M 17 TGNN + Q192S + L193M + N452P 2.3

As is clear from the tables, the modified PQQGDH of the present invention in all cases exhibited a reactivity for glucose that was higher than that for maltose.

Example 4 Fabrication and Evaluation of an Enzyme Sensor

20 mg carbon paste was added to 5 units of the modified PQQGDH of the present invention and freeze-dried. After thorough mixing, it was filled onto the surface of a carbon paste electrode that carries approximately 40 mg carbon paste, and polished on filter paper. The electrode was treated for 30 minutes at room temperature in 10 mM MOPS buffer (pH 7.0) containing 1% glutaraldehyde, and then treated for 20 minutes at room temperature in 10 mM MOPS buffer (pH 7.0) containing 20 mM lysine in order to block the glutaraldehyde. The electrode was equilibrated for at least one hour at room temperature in 10 mM MOPS buffer (pH 7.0). The electrode was stored at 4° C.

The glucose concentration was measured using the enzyme sensor prepared above. Glucose was quantitatively measured in the range from 0.1 mM to 5 mM using the enzyme sensor having immobilized the modified PQQGDH of the present invention.

Example 5 Measurement of the Enzymatic Activity Using a Purified Enzyme Preparation

A cation-exchange chromatography column packed with TSKgel CM-TOYOPEARL 650M (Tosoh Corporation) was equilibrated with 10 mM phosphate buffer at pH 7.0 and the wild-type crude enzyme or the modified PQQGDH crude enzymes obtained in Example 2 was adsorbed on the column. The column was washed with 750 mL 10 mM phosphate buffer (pH 7.0) and the enzyme was then eluted with 10 mM phosphate buffer (pH 7.0) containing from 0 to 0.2 M NaCl. The flow rate was 5 mL/minute. The fraction exhibiting GDH activity was collected and was dialyzed overnight against 10 mM MOPS-NaOH buffer (pH 7.0) to obtain an electrophoretically-homogeneous modified PQQGDH protein. The enzymatic activity of the purified enzyme preparations for glucose and maltose was measured as in Example 4. Typical results are shown in the following table.

TABLE 20 ENZYMATIC ACTIVITY Mal:Glu (Glu; 4 mM) (%) MODIFIED PQQGDH (U/mg) 4:4 40:4 TGSNN + Q192G + L193E 357 4.1 28* TGSNN + Q192S + N452P 723 3.5 19* TGSNN + Q192G + L193E + 213 0.56   5.7 N452P TGSNN + Q192S + L193M + 285 2.0 16* N452P TGSNN + Q192G + L193E + 381 0.99   8.92 A318K + N452P TGSNN + Q192G + L193E + 469 0.55   6.74 A318Q + N452P *measured at Mal:Glu = 50:5

In addition, the enzymatic activity for glucose and maltose of the modified PQQGDH having the TGSNN+Q192G+L193E+N452P mutations is shown in FIG. 1.

Example 6 Comparison with Prior-Art Modified PQQGDHs

The enzymatic activity for glucose and maltose was measured as in Example 4 using the wild-type crude enzyme obtained in Example 2, the modified PQQGDH crude enzymes obtained in Example 2, and, for comparison, prior-art modified PQQGDHs lacking the mutation at amino acid residue G at Position 99 of PQQGDH but having the 1 or 2 or more amino acid substitutions at other positions.

The following table shows typical results for the enzymatic activity of the modified PQQGDH of the present invention having various mutations and substitution of 99G with TGSNN, a comparative PQQGDH having the same mutation(s) but not substitution at 99G and wild-type PQQGDH. The results are expressed by the activity for 4 mM glucose or 10 mM glucose and for the ratio of the activity for maltose to the activity for glucose (%, 4 mM maltose:4 mM glucose or 10 mM:10 mM).

TABLE 21 ACTIVITY FOR Mal/Glu GLUCOSE (U/mg) (%) 4 mM 10 mM 4:4 10:10 WILD TYPE 47.69 86.96 73.5 75.5 TGSNN + Q192G + L193E 10.63 20.20 4.2 5.1 Q192G + L193E 6.11 11.62 9.0 10.5 TGSNN + Q192G 15.90 26.36 9.1 11.9 Q192G 19.30 31.79 20.2 26.1 TGSNN + Q192A 26.26 48.56 7.8 9.9 Q192A 21.44 41.32 15.2 18.2 TGSNN + Q192S 35.83 60.94 12.9 17.0 Q192S 26.66 44.91 28.0 37.8 TGSNN + N452P 58.71 110.01 16.3 18.7 N452P 56.53 103.43 26.4 33.1 TGSNN + Q192S + N452P 37.98 72.23 1.5 2.2 Q192S + N452P 28.93 56.12 5.0 6.6

As shown in the table, the enzymatic activity of the modified enzymes according to the present invention with the insertion of the TGSNN sequence for glucose is either maintained or increased compared to the corresponding modified enzymes having the same mutations but lacking the insertion of TGSNN. In addition, the maltose-versus-glucose activity ratio is reduced to about one-half to one-fifth, and the selectivity for glucose is thus improved. For example, it has already been reported that the substrate specificity is increased in the modified PQQGDH having a double mutation Q192G/L193E. The activity of the Q192G/L193E modified PQQGDH for 4 mM glucose was 6.1 U/mg and the 4 mM maltose-versus-4 mM glucose activity ratio was 9%. In contrast, the additional insertion of TGSNN provided an approximately 1.7-fold increase in the activity for 4 mM glucose to 10.63 U/mg, and a 2.14-fold increase in substrate specificity, with 4.2% for the 4 mM maltose-versus-4 mM glucose activity ratio.

As demonstrated by these results, the modified PQQGDH of the present invention has a high enzymatic activity for glucose and a high substrate specificity for glucose over maltose.

INDUSTRIAL APPLICABILITY

The present invention is useful for assaying glucose and particularly for measuring blood sugar level. 

1. A modified pyrroloquinoline quinone glucose dehydrogenase, wherein the amino acid residue G at Position 99 of a pyrroloquinoline quinone glucose dehydrogenase (PQQGDH) represented by SEQ ID NO: 1 or the amino acid residue G at Position 100 of the PQQGDH represented by SEQ ID NO: 3 is substituted by the amino acid sequence TGZN (where Z is SX, S, or N and X is any amino acid residue), and wherein from 1 to 10 of amino acid residues at Positions 1 to 98 or at Positions 100 to 478 of SEQ ID NO: 1, or amino acid residues at Positions 1 to 99 or at Positions 101 to 480 of SEQ ID NO: 3, may be substituted by any other amino acid residue(s).
 2. The modified pyrroloquinoline quinone glucose dehydrogenase according to claim 1, wherein Z is SX (where X is any amino acid residue).
 3. The modified pyrroloquinoline quinone glucose dehydrogenase according to claim 1, wherein Z is SN.
 4. The modified pyrroloquinoline quinone glucose dehydrogenase according to claim 1, further comprising one or more mutations selected from the group consisting of the following amino acid substitutions: Q192G, Q192A, or Q192S; L193X (where X is any amino acid residue); E277X (where X is any amino acid residue); A318X (where X is any amino acid residue); Y367A, Y367F, or Y367W; G451C; and N452X (where X is any amino acid residue).
 5. The modified pyrroloquinoline quinone glucose dehydrogenase according to claim 4, comprising any of the following combinations of amino acid mutations: G99(TGSXN)+Q192G+L193E; G99(TGSXN)+Q192S+N452P; G99(TGSXN)+Q192G+L193E+N452P; G99(TGSXN)+Q192S+N452P; G99(TGSNN)+N452P; G99(TGSNN)+Q192G+L193E+N452P; G99(TGSNN)+Q192S+N452P; G99(TGSNN)+Q192G+N452P; G99(TGSNN)+L193E+N452P; G99(TGSNN)+Q192S+L193M+N452P; G99(TGSNN)+A318Y+N452P; G99(TGSNN)+Q192G; G99(TGSNN)+Q192S; G99(TGSNN)+Q192A; G99(TGSNN)+Q192G+L193E; G99(TGSNN)+Q192S+L193X; G99(TGSNN)+Q192S+L193M; G99(TGSNN)+Q192S+L193T; G99(TGSNN)+Q192S+E277X; G99(TGSNN)+Q192S+N452X; G99(TGSNN)+Q192S+L193X+A318Y+N452P; G99(TGSNN)+Q192S+A318X+N452P; G99(TGSNN)+Q192G+L193E+A318X+N452P; G99(TGSKN)+Q192S+N452P; G99(TGSRN)+Q192S+N452P; G99(TGSWN)+Q192S+N452P; G99(TGSN)+Q192S+N452P; G99(TGSN)+Q192G+L193E; G99(TGSN)+Q192S+L193M; G99(TGNN)+Q192S+N452P; G99(TGNN)+Q192G+L193E; or G99(TGNN)+Q192S+L193M (where X is any amino acid residue).
 6. The modified pyrroloquinoline quinone glucose dehydrogenase according to claim 5, comprising any of the following combinations of amino acid mutations: G99(TGSXN)+Q192G+L193E; G99(TGSXN)+Q192S+N452P; G99(TGSNN)+Q192G+L193E+N452P; G99(TGSNN)+Q192S+L193M+N452P; G99(TGSNN)+Q192G+L193E+A318K+N452P; or G99(TGSNN)+Q192G+L193E+A318Q+N452P.
 7. A gene coding for the modified pyrroloquinoline quinone glucose dehydrogenase according to claim
 1. 8. A recombinant vector, comprising the gene coding for the modified pyrroloquinoline quinone glucose dehydrogenase according to claim
 7. 9. A transformant or transfectant transformed by the recombinant vector according to claim
 8. 10. A method of preparing a modified pyrroloquinoline quinone glucose dehydrogenase, comprising: culturing the transformant according to claim 9, and recovering a pyrroloquinoline quinone glucose dehydrogenase from the culture.
 11. A method for analyzing glucose level using the modified pyrroloquinoline quinone glucose dehydrogenase according to claim
 1. 12. A glucose assay kit comprising the modified pyrroloquinoline quinone glucose dehydrogenase according to claim
 1. 13. An enzyme electrode comprising the modified pyrroloquinoline quinone glucose dehydrogenase according to claim
 1. 14. A glucose sensor comprising the enzyme electrode according to claim 13 as a working electrode. 